It is becoming more common for computers and other electronics to utilize low power quiescent modes during periods of substantial inactivity. For example, desktop and laptop computers can be placed into standby and hibernation modes in which the computer is essentially shut down and not able to function until manual switching of the computer back into its fully active state. Another power management approach short of a system-wide idling of the computer is to selectively power down only certain components, such as the computer's hard disk drive. However done, these approaches are commonly used on consumer computers, but may not be practical for use by servers where the rapid transitions and brief intervals of activity make it difficult to conserve idle power. Yet, as much as 60% of the power consumed by servers in a typical data center is wasted by idle servers that are powered on, but not performing useful work. The recent trend towards server consolidation is partly motivated by the high energy cost of idle systems. By moving services to virtual machines, several services can be time-multiplexed on a single physical server, increasing average utilization. Consolidation allows the total number of physical servers to be reduced, thereby reducing idle inefficiency. However, server consolidation by itself does not close the gap between peak and average utilization—data centers still require sufficient capacity for peak demand, which can leave some servers idle in the average case. Furthermore, consolidation does not save energy automatically—system administrators must actively consolidate services and remove unneeded systems.
Although support for sleep states is widespread in handheld, laptop and desktop machines, as noted above these states are not typically used in current server systems. The Advanced Configuration and Power Interface (ACPI) standard defines a rich set of power states that an operating system and hardware platform can use to manage power consumption. These low power states operate by transferring volatile data (e.g., main memory) to non-volatile storage (typically disk). Unfortunately, the high restart latency of these states renders them unacceptable for interactive services—disks simply provide insufficient I/O response time and bandwidth to restore memory state in fractions of a second. Moreover, unlike consumer devices, servers cannot rely on the user to transition between power states; they must have an autonomous mechanism that manages state transitions.
Recent server processors include CPU throttling solutions (e.g. Intel Speedstep™, AMD Cool‘n’Quiet™) to reduce the large overhead of light loads. These processors use dynamic voltage and frequency scaling (DVFS) to reduce their operating frequency linearly while gaining cubic power savings. DVFS relies on operating system support to tune processor frequency to instantaneous load. In Linux, the kernel continues lowering frequency until it observes ˜20% idle time. Improving DVFS control algorithms remains an active research area. Nonetheless, DVFS can be highly effective in reducing CPU power. However, as FIG. 1 shows, CPUs account for a small portion of total system power.
Energy proportional computing seeks to extend the success of DVFS to the entire system. In this scheme, each system component is redesigned to consume energy in proportion to utilization. In an energy-proportional system, explicit power management is unnecessary, as power consumption varies naturally with utilization. However, as many components incur fixed power overheads when active (e.g., clock power on synchronous memory busses, leakage power in CPUs, etc.) energy-proportional operation may not be readily achievable.
Another more recent concern is the AC to DC conversion losses in computer systems large and small, and this has led to a variety of research proposals, product announcements, and standardization efforts to improve power supply efficiency. The concern is particularly applicable to data centers, server farms and other multi-server systems such as can be implemented using blade servers, where each watt wasted in the power delivery infrastructure implies even more loss in cooling. As in conventional blade enclosures, power is provided by multiple PSUs connected in parallel. A conventional load-sharing IC continuously monitors and controls the PSUs to ensure load is divided evenly among them. Individual PSUs can be disabled and electrically isolated when they are not needed to supply the load.
A variety of recent initiatives seek to improve server power efficiency:
1. 80+ certification. The EPA Energy Star program has defined the “80+” certification standard to incentivize PSU manufacturers to improve efficiency at low loads. The 80+ incentive program is primarily targeted at the low-peak-power desktop PSU market. 80+ supplies carry an average 30% cost premium and require considerably higher design complexity than conventional PSUs, which may pose a barrier to widespread adoption in the reliability-conscious server PSU market. Furthermore, despite their name, the 80+ specification does not require energy efficiency above 80% across all loads, rather, only within the typical operating range of conventional systems.
2. Single voltage supplies. Unlike desktop machines, which require five different DC output voltages to support legacy components, server PSUs typically provide only a single DC output voltage, simplifying their design and improving reliability and efficiency.
3. DC distribution. Recent research has called for distributing DC power among data center racks, eliminating AC-to-DC conversion efficiency concerns at the blade enclosure level. However, the efficiency advantages of DC distribution are unclear and deploying DC power will require multi-industry coordination.
4. Dynamic load-sharing. Blade enclosures create a further opportunity to improve efficiency through dynamic load-sharing. Hewlett Packard's Dynamic Power Saver feature in the HP Blade Center c7000 employs up to six high efficiency 2.2 kW PSUs in a single enclosure, and dynamically varies the number of PSUs that are engaged, ensuring that all active supplies operate in their “green” zone while maintaining redundancy.